1. Field of Invention
The invention relates to microwave intrusion detectors, and more specifically to such detectors with adjustable alarm thresholds.
2. Description of the Prior Art
Intrusion detection systems based on microwave technologies typically include a Doppler signal detector directed toward a region under surveillance. The detector may include a microwave transceiver that transmits energy toward and receives reflected energy from the region. When the reflected energy is returned from a moving object, such as an intruder, it is shifted in frequency by a well known phenomena referred to as the Doppler effect. Mixing the returned energy with the transmitted energy produces a Doppler signal equal in frequency to the shift.
The Doppler signal is concentrated by amplification and filtering and is compared to a reference. Signals that exceed the reference are processed to reduce false alarms. Processed signals determined to represent valid alarms then activate appropriate annunciators and solenoids to warn of the intrusion.
As microwave detectors have become more sophisticated, features have been added to accommodate a variety of installation conditions. One such feature is a noise compensating circuit that actively adjusts the alarm threshold for the characteristics of a particular installation. The circuit typically includes a peak detector and an integrator having rise times and decay parameters that actively maintain the alarm threshold above Doppler signals caused by certain background activity. Signals from moving curtains, for example, increase the threshold, and are masked, without setting off the alarm.